Steam drive for incompetent tar sands



Aug. 13, 1968 P. VAN MEURS ET AI- s'rEAM DRIVE FOR INCOMPETENT TAR sANDsFiled Sept. 9, 1966 r. lA /1 FIG.

FIG. 2c

FIG. 2b

FIG. 2o

INVENTORSZ P. VAN MEURS C. W. VOLEK BYfA/ qc THEIR AGEN United StatesPatent O 3,396,791 STEAM DRIVE FOR INCOMPETENT TAR SANDS Pieter VanMeurs and Charles W. Volek, Houston, Tex., assignors to Shell OilCompany, New York, N.Y., a corporation of Delaware Filed Sept. 9, 1966,Ser. No. 578,245 6 Claims. (Cl. 166-11) This invention relates toproducing a petroleum material from a subsurface reservoir that containsa viscous petroleum material, such as a tar or a viscous oil. Moreparticularly, the invention relates to an improved steam drive processfor recovering a normally viscous oil from a reservoir that isimpermeable at the reservoir temperature and is incompetent at atemperature that thermally mobilizes the reservoir oil. As used hereinin reference to viscous petroleum reservoirs, the term incompetentrefers to a petroleum bearing earth formation, which may or may not becompetent at the reservoir temperature, which tends to slump or caveinto holes or fractures in the formation at a temperature at which thereservoir petroleum is thermally mobilized to an extent making it apumpable liquid.

A previously conceived process for producing petroleum material from areservoir that is impermeable at reservoir temperature and incompetentat a temperature that mobilizes the reservoir petroleum is described inU.S. Patent No. 3,221,813. In the patented process a fracture isextended between at least one pair of injection and production Wells. Areservoir heating il-uid, preferably steam, is flowed through thefracture at a pressure suicient to separate the walls of the fracture.During the steam circulation, since the ilow path between the fracturewalls tends to become plugged with viscous petroleum material, higherand higher pressures must be applied in order to circulate the steam.The steam circulation is periodically interrupted in order to remove theviscous plugs and avoid the need for applying excessive pressures whilethe wells and flow paths are lled with pressurized steam. The viscousplugs are removed by circulating a tar-entraining liquid since theliquid can, if necessary, be injected safely at pressures that may reachthe formation fracturing pressure. While steam is circulating, itremoves viscous petroleum material from the fracture walls and leavespetroleum-material-depleted permeable sand. The permeable layer of sandgrows vertically until it has expanded through substantially the fullvertical extent of the reservoir.

4Objects of the present invention include the following:

An improved process is provided for producing a thermally mobilizedmaterial from a viscous petroleum material reservoir that issubstantially impermeable at reservoir temperature and incompetent at atemperature at which the reservoir petroleum is thermally mobilized.

A process is provided for heating an incompetent reservoir formation bycirculating heated fluid through a horizontal fracture within thereservoir wlhile correlating the iluid composition, the fluid ilow rate,and the rate at which the iluid temperature is raised above thereservoir temperature, so that an adequate rate of flow is maintained atpressures that remain well below the fracturing pressure.

A process is provided for eiciently producing petroleum from anincompetent reservoir formation by adjusting the ratio of productionWells to injection wells and the production well bottom hole pressureswhile circulating yhot aqueous fluid. The fluid composition, flow rateand fluid heating rate are adjusted as required to maintain a pressurewithin the reservoir which is substantially as low as the steam pressurecorresponding to the temperature selected as that to which the reservoirpetroleum is heated in order to reduce its viscosity.

ICC

In general, in accordance with the present invention, petroleum isproduced from a subsurface viscous petroleum reservoir lwhich issubstantially impermeable at reservoir temperature, substantiallyincompetent at a temperature that thermally mobilizes the reservoir Apetroleum and is penetrated by at least one pair of wells which areinterconnected by a horizontal fracture within the reservoir. In thepresent process, a heated aqueous liquid is circulated through ahorizontal lfracture at a pressure greater than the overburden pressurewhile the composition of the liquid, the temperature of lthe liquid, andthe duration of its circulation, are correlated to form a permeablelayer of petroleum-depleted sand at a temperature that thermallymobilizes the reservoir petroleum. In accomplishing this, the aqueousliquid is softened by the increasing amounts required to provide anaqueous liquid that is non-scaling at the temperature to which it isheated. The heating temperature is increased to one causing areservoir-petroleum-mobilizin'g temperature to be reached in theentrainment of petroleum to occur, in the circulating aqueous liquid allalong the flow path between the wel-ls. The circulation is continueduntil the petroleum extraction has formed a permeable layer of sand thathas a temperature at which the reservoir petroleum is mobile and has avolume that can convey an adequate rate of ow at an injection pressureless than the overburden pressure. Steam is then circulated through thereservoir at a relatively low pressure and the permeable path isexpanded to vertically sweep the reservoir with substantially noplugging, due to the depositing of viscous petroleum materials or scale,having been allowed to occur Within the flow path.

FIGURE l illustrates a vertical section of a viscous petroleum reservoirthat has been penetrated by wells Iwhich are suitable for use in thepresent invention.

FIGURES 2-A through 2-C are views illustrating the portion of thereservoir that surrounds the injection well during successive stages ofheating operations in which hot fluids are circulated through thereservoir.

Referring to FIGURE l, the drawing shows a portion of a typical patternof injection and production wells in which an incompetent viscouspetroleum reservoir formation of the type typified by the Athabasca tarsand, is penetrated by the wells 2, 3 and 4. Well 3 is equipped as aninjection well which contains a casing string 4a, casing head 5,injection tubing string 6, packer 7, and casing perforations 8. Wells Zand 4 are equipped as production wells which contain casing strings 9,production tubing strings 10, and casing perforations 11. Communicationbetween the wells is established by a horizontal fracture 12.

In certain situations, such a fracture can be formed by injecting andpressurizing a fluid that is pumped through tubing string 6 andperforations 8. When the pressure becomes sufficient to form ahorizontal fracture, such as fracture 12, and separate the fracturewalls by lifting the overburden 13, the fluid flows on through thefracture and into the perforations 11 and the production tubing strings10 of the production wells 2` and 4. If a fluid which is so circulatedis heated to a temperature that mobilizes the reservoir petroleummaterial, the mobilized petroleum tends to become entrained in andextracted by the circulating fluid. As long as the entrained petroleumstays mobile until it reaches the production well, the flow lpathremains unplugged and tends to expand as more petroleum is extracted toleave more petroleumdepleted permeable sand.

A steam such as a dry or low quality steam is a particularly suitableheated uid to be circulated through such a reservoir. However, as isdiscussed in U.S. Patent No. 3,221,813, it is necessary to periodicallyinterrupt the steam circulation in order to effect a liquid extractionof petroleum materia-ls that have cooled and form viscous plugs withinthe flow path that interconnects the Wells.

It has now :been discovered that such a plugging of such a flow path canbe substantially completely avoided. In order to do this, the initialportion of lthe heating operation is conducted by circulating hot waterthrough the reservoir. At this stage the water-circulating heatingprocess is advantageous. However, such :a heating procedure would becometoo expensive if it were to be utilized to heat all of the swept zonewithin the reservoir interval. The hot water heating is accomplished byheating the water at a surface location and circulating it through thereservoir at a rate sufficient to transport heat fr-om the surfacelocation to the openings into the production well. This requires themaintenance of a flow rate that is at least about lo barrel per minute,in a relatively shallow reservoir in which there is a relatively lowheat loss in the injection tubing string. Higher rates are required inrespect to deeper reservoirs or situations involving higher heat losses.

We have furthe-r discovered that the water which is used must besoftened by the increasing amounts required to yield an aqueous liquidthat is non-scaling at the temperature to which it is heated.

The heating and circulating of hot water are continued until the fluidthat enters the production well is at leas-t hot enough to thermallymobilize the reservoir petroleum. They are preferably continued untilthe temperature of fluid entering the production well is suflicient toreduce the viscosity of the reservoir petroleum material to less thanabout 50 cp. When, or relatively soon rafter, such a ternperature hasbeen attained, the fluid being circulated through the reservoir isconverted from liquid t-o steam.

FIGURE 2-A shows the region around the injection well 3 during an earlystage of heating the reservoir by injecting steam into fracture 12. Insuch an operation the steam tends to condense and yield most of its heatnear the well. This is indicated by dotted region 14, in which thevariation in vertical distance away from fracture 12 represents thevariation in the temperature with horizontal distance away from well 3.As the steam leaves the well, the reservoir petroleum which is thermallymobilized and swept along with the flow of steam moves from a hot regioninto a region lthat is relatively cool. The cooling petroleum tends toform a viscous plug before it reaches the production well.

FIGURES 2-B shows the region .around injection well 3 during an earlystage of heating the reservoir formation by the present process. In suchan operation, heated aqueous liquid is circulated between the injectionand production wells at a flow rate such that heat is transported fromthe surface located heater to the openings into the production well.This causes the heat to be distributed substantially uniformly along thefracture in the manner indicated in section by the dotted region 14a.

Such a heating of a layer of subsurface earth formation causes a liftingof the overburden, as is indicated by the i dotted and solidrepresentations and and Sa of the wellhead 5. However, in the presentprocess, such a lifting of the overburden causes no significant increasein the pressure required to separate the walls of fracture 12 andcirculate the liquid through the reservoir. Such injection andproduction wells are usually separated by significant distances, andeven Where they are separated by as little as 25 feet, such a heatedzone would have an area of nearly 2000 square ft. The force required tobend the overlying strata by an amount sufiicient to accommodate thelifting of the overburden would be insignificant in respeet to the forceproduced by applying the overburden pressure to an area of more than2000 square feet.

FIGURE 2C shows the region around injection well 3 during a later stageof heating the reservoir formation by the present process. In this stagethe fracture walls have been moved apart as indicated at 12a and 12b.The space between the fracture walls is occupied by petroleum-depletedpermeable sand 15. A general characteristic of viscous petroleumreservoirs that are substantially impermeable at reservoir temperature4and inc-ompetent at a temperature that mobilizes the reservoirpetroleum is that, when such a reservoir formation is depleted ofpetroleum, it becomes a permeable sand.

In operating the present process, as soon as the fracture walls havebecome heated to a temperature that mobilizes the reservoir petroleum,the mobilized petroleum begins to be entrained in and extracted by thecirculating hot water. Since the temperature is uniform alongsubstantially the entire distance -between the injection and productionwells, the entrained petroleum encounters no cool zone in which it canbecome a viscous plug.

After the flow path through the reservoir has been heated to atemperature that mobilizes the reservoir petroleum, the circulatingheated fluid is converted from liquid to steam.

When the volume of petroleum-depleted permeable sand is sufficient toconvey fluid at an adequate rate of flow at an injection pressure ofless than the overburden pressure, the injection pressure is preferablyreduced to such a lower pressure. This allows the weight of theoverburden to rest on the layer of permeable sand. Since this sanddiffers from the other portion of the reservoir sand only in the absence`of petroleum, it does not tend to become embedded in the walls of thefracture. As the fracture walls are converted to the permeable sand, thefracture is, in effect, replaced by a permeable path that consists of alayer of petroleum-depleted permeable sand.

Steam is preferably circulated through the permeable path atsubstantially the steam pressure corresponding to a temperature at whichthe reservoir petroleum is thermally mobilized. A back pressure withinthe production wells is desir-able only if necessary in order tomaintain a steam pressure corresponding to a selected temperature. Ingeneral, the flow resistance within the permeable path through thereservoir provides suicient back pressure when the flow rate isrelatively high. It is generally preferable to circulate the steam at arelatively high rate at relatively low pressure by (1) providing atleast about four .production Wells that are responsive to each injectionwell (2) injecting the steam at a rate such that at least some livesteam is transported through the reservoir and into substantially :allrof the production wells and (3) maintaining a low bottom hole pressurein each production well by pumping fluid out of the well at a ratesufficient to produce at least some live steam. In reservoir Kformationsthat are similar the Athabasca tar sand, a particularly suitable steaminjection rate is a rate in the order of about 10 to 20 tons per acreper day.

In heating and circulating :an aqueous liquid in the present process,the aqueous liquid can comprise substantially any liquid that is inertat the temperature to which it is heated. Pure water, brine, or brackishWater, can be used. However, it is essential that the :aqueous liquidwhich is used be softened as required to provide a liquid that issubstantially non-scaling at the temperature to -which it is heated.Omitting such a softening allows scale to form in the tubing string andthe fracture and/or the layer of oil-depleted permeable sand that isbeing formed within the fracture. Such a scaling decreases thepermeability and, las the permeability is decreased, higher pressuresare required to maintain an adequate rate of ow. Such ef- -fectsaccumulate until the injection pressure reaches a fraoturing pressureand forms a new fracture, which opens a new flow path that bypassesportions of the reservoir that were previously heated. The watersoftening procedure can utilize substantially any of the precipitation,ion-exchange, chelation, or the like techniques that are known to thoseskilled in the art for keeping the alkaline earth metal ions fromprecipitating to form scale at elevated temperatures. A particularlysuitable Water softening technique, which is designed to prevent scalingiat the temperatures -used in numerous thermal processes for producingoil, is described in U.S. Patent No. 3,193,009.

When a temperature that exceeds 212 F. has been attained within thefracture through which heated aqueous liquid is circulated in thepresent process, the fluid being circulated `through the fracture ispreferably converted from a liquid to steam. Such a conversion ispreferably initiated las soon as the fracture walls have been heated toa temperature above 212 F. that reduces the viscosity of the reservoirpetroleum to a relatively low value, such as less than about 50 cp. Therate at which the conversion is alected is critical in the sense that itshould be slow enough to avoid any localized cool spots lwithin the flowpath that interconnects injection and production Wells. During theconversion, the steam temperature is preferably kept equal to that ofthe hot wate-r, with the changeover being gradual, rather `thanincremental.

The conversion from hot water to steam is preferably affected byincreasing the residence time of the aqueous iiuid in the heater. Thiscauses increasing proporitons of the fluid to be converted from liquidto steam at the same temperature and pressure that were being impartedto the heated aqueous liquid. The rate at which the liquid is convertedto steam is preferably monitored by determining the temperature of fluidentering the production well and keeping the rate low enough so thatsubstantially no decrease occurs in that temperature. Suchdeterminations can be made conveniently with conventional downhole, orsurface located, temperature measuring devices. Where the surfacelocated devices are used, it is, of course, preferable to calibrate forthe changes in temperature that occur during the passages through theWell.

The convers-ion from hot water to steam can either precede or follow thereduction of the injection pressure to less than overburden pressure. Itis generally preferable to reduce the pressure while circulating hotwater in yorder to allow the weight of the overburden to settle onto thelayer of permeable petroleum depleted sand before converting from hotwater to steam. This avoids the need for a boiler capacity sufficient togenerate steam at a relatively high rate at a pressure greater than theoverburden pressure. In addition, when the pressure is lowered beforeinitiating the conversion, the pressure reduction :and the conversionfrom water to steam is generally smoother and leads to an earlierproduction of significant amounts of oil.

In various viscous petroleum reservoirs which are substantiallyimpermeable vat reservoir temperature and incompetent at a temperaturethat thermally mobilizes the reservoir petroleum, horizontal fracturesform when conventional fracturing procedures are employed. In reservoirswhich are normally biased toward horizontal fracturing the present oilrecovery process can be initiated by opening a pair of wells into thereservoir and interconnecting them via a horizontal fracture that isformed by means of known procedures. In reservoirs in which the regionaltectonics are such that only vertical fractures are formed whenconventional fracturing procedures are employed, or in reservoirs inwhich the fracturing tendencies are unknown or are likely to producevertical fractures, the present oil -recovery process is preferablyinitiated by: opening a pair of boreholes into the reservoir, thermallybiasing the reservoir to form a horizontal fracture, and theninterconnecting the wells by forming a horizontal fracture within thethermally biased reservoir. In thermally biasing a reservoir to form ahorizontal fracture that extends away from a Well, the reservoir isiirst vertically fractured at the reservoir temperature. Liquid is thenpumped into the fracture at a rate adapted to transport heat from asurface location to the fracture. The inflowing liquid is heated totemperatures that are made increasingly greater than the reservoirtemperature while increasing the injection pressure as required tomaintain the flow rate. The liquid injection is continued, while thetemperature and pressure are so increased, until a horizontal fractureis formed. Such a thermal biasing procedure is described in greaterdetail in the eo-pending application of C. S. Matthews, P. Van Meurs andC. W. Volek, Ser. No. 578,533, filed Sept. 12, 1966.

Example I.-0l production Field tests in the Athabasca tar sand havedemonstrated that commercial oil production can be obtained bycirculating steam through such a tar sand. In the tests, a permeablepath was formed by extending a horizontal fracture between a pair ofwells. During the circulation of steam, recurrent plugging of the tiowpath necessitated interruptions of the steam circulation w'hile a tarentraining liquid was circulated in order to remove the plugs. The flowpath through which the steam was circulated expanded vertically and thevertical sweep efficiency of the oil production operation was good.

A linear model was prepared to simulate a portion of a reservoir, havingthe characteristics of an Athabasca tar sand, between rows of Wells forinjecting and producing tiuids. The model was filled with inert granularmaterial from which water was displaced by oil until only a residualwater `saturation was left in the granular material. A horizontalfracture that interconnected a pair of Wells was mechanically opened atthe bottom of the model reservoir.

Scaled experiments with the model demonstrated that, in the modeledreservoir, as in the field, plugging occurred when dry steam or lowquality steam was circulated through the fracture before heat had beenimparted to an extensive region within the reservoir. In both the modelreservoir and the eld reservoir the occurrence of plugging was indicatedby simultaneous occurrences of a .pressure increase while fluid wasbeing injected at constant rate and a decrease in the temperature of thefluid that entered the production well. Such events indicate thatcommunication between the injection and production wells is being lost.It was observed, in both the model and the eld reservoir, that suchplugging materials could be rcrnoved by interrupting the circulation ofsteam and injecting a slug of hot liquid tar-entraining material whileapplying sufficient pressure to force a circulation of thetar-entraining material through the flow path from the injection well tothe productoin well.

A series of additional tests with the model reservoir formationindicated the suitability of two different methods for preventingplugging. In the first, the heating was initiated by circulating hotwater until the temperature of the production well was raised to such alevel that the tar became mobile. Then low-quality steam injection wasstarted. Both during hot-water injection and lduring lowquality steaminjection, the B.t.u. injection rate was kept constant at the maximumcapacity of the boiler. During the next phase of the process the qualityof the injected steam was raised, again at constant heat injection rate,in such a manner that the temperature of the production well did notdecrease. When the temperature of the production well reached steamtemperature the fracture was gradually closed mechanically, starting atthe production side. In the eld the fracture can be closed by decreasingthe backpressure on the production well. The fracture will be completelyclosed when the injection pressure has dropped below overburdenpressure.

In the second method, as in the rst, injection was started with hotwater until the temperature of the production well was high enough tomobilize the tar. Then the fracture was gradually closed while theinjection of hot water was continued. The closing of the fracture wascarried out at such a rate that the temperature of the puoduction Welldid not decrease. When complete closure of the fracture was obtained, a|changeover was made from the injection of hot water to steam of agradually increasing quality. Again the steam quality was raised in sucha manner that the temperature of the production well did not decrease.

Comparison of the two methods showed that the second method, rin whichthe fracture was closed before steam injection was started, workedsmoother and showed an earlier oil production response than the first,in which the fracture was heated with steam before being closed.

Example II.-F luid composition control The importance of softening theaqueous liquid that is heated and circulated through a horizontalfracture and the feasibility of slowly converting the circulating fluidfrom hot `aqueous liquid to steam has been demonstrated in Ifield tests.A pattern of injection and production wells was completed in a shallowlayer of Missouri tar sand and an interconnecting horizontal fracturewas extended from the injection well to the production Wells at a depthof 300 feet. The fracturing pressure was in the range of 800- 1000p.s.i., and cold water could be circulated through the fracture at aboutbbl/minute at a bottom hole pressure of 410 p.s.i.g.

The water used was obtained from a nearby source well and was heated bymixing it with steam. As the so heated water was circulated through thefracture, the pressure required to maintain the circulation rate soonbegan to rise. Remedial treatments, including the propping of thefracture, failed to eliminate the need for increasing the pressure inorder to maintain the circulation rate. It Was later discovered that thescale-depositing tendency of the water increased with temperature, andthe increase in the pressure Lrequirement was due to the tubing stringand fracture becoming plugged with scale,

The Wells were acidized to reduce the plugging effects of the scaledeposit in respect to circulating a liquid through the fracture. Thesame water was softened by process of the type described in U.S. PatentNo. 3,193,409 to the extent required to provide a liquid that depositedno scale at the temperature to which it was heated. The initial portionsof softened water were heated in the manner described above to atemperature estimated to have remained in the fracture and thetemperature of the circulating water was increased at a rate of about 50per day. The heating and circulating were continued until water wasbeing circulated at 475 F. at a pressure Well below the formationfracturing pressure.

While continuing to circulate the hot Water at 475 F., the rate at whichthe water owed through the heater and fracture was gradually throttledback to half the initial rate. The rate reduction was affected by overabout a three-day period, so that it caused a longer residence time inthe heater and without causing any significant change in the temperatureor pressure of the fluid injected into the fracture. Thus, thecirculating uid was gradually converted from liquid to steam. Thequality of the steam was then gradually increased to a steam quality ofabout 60 percent. This conversion from liquid to steam caused nodecrease in the temperature of the 4fluid that entered a monitor wellfeet from the injection well.

We claim as our invention:

1. A process for producing petroleum from a viscous petroleum reservoirthat is substantially impermeable at reservoir temperature, is`substantially incompetent at a temperature that thermally mobilizes thereservoir petroleum and is penetrated by at least one pair of wells thatare interconnected by a horizontal fracture extending through thereservoir, which process comprises:

(a) heating aqueous liquid at a surface location and circulating theheated liquid through the fracture between said pair of wells whilemaintaining a back pressure that at least substantially equals theoverburden pressure and an injection pressure that causes ow at a rateadequate for transferring heat from the surface location to the openingsinto the production well;

(b) increasing the temperature of the circulating heated aqueous liquiduntil fluid owing into the production well has a temperature thatthermally mobilizes the reservoir petroleum while increasing thesoftness of the aqueous liquid to the increasing extent required toprovide a liquid that is nonscaling at the temperature to which theliquid is being heated;

(c) forming a permeable path Within the reservoir by entraining thethermally mobilized reservoir petroleum in the circulating heatedaqueous liquid and thus converting a portion of the reservoir to a layerof petroleum-depleted permeable sand;

(d) reducing the pressure within the reservoir to less than overburdenpressure and circulating steam through the reservoir at a temperature atwhich said reservoir petroleum is mobilized and entrained;

(e) recovering said petroleum from fluid that has circulated through thereservoir.

2. The process of claim 1 wherein:

(a) the rate at which fluid is circulated through the reservoir is atleast greater than 1/10 bbl/minute; and

(b) the temperature to which the circulating uid is heated is increasedto at least a temperature at which the reservoir petroleum viscosity isless than 50 cp.

3. The process of claim 1 wherein:

(a) the circulating of heated aqueous liquid is continued during andafter said reduction of the pressure within the reservoir to less thanoverburden pressure; and

(b) the conversion of the circulating fluid from liquid to steam iseffected so gradually that no significant temperature reduction occursin the uid that enters the production Well.

4. The process of claim l wherein:

(a) the reservoir is an incompetent tar sand; and

(b) the rate at which steam is circulated through the permeable pathWithin the reservoir is in the order of l0 to 20 tons per acre per day.

5. The process of claim 1 wherein:

(a) the reservoir is one in which fractures that are formed at reservoirtemperature are vertical fractures;

(b) the reservoir is intially vertically fractured at reservoirtemperature; and

(c) aqueous liquid is injected into the fracture formed at reservoirtemperature While relatively slowly in- -creasing the temperature of theinjected aqueous liquid and increasing the injection pressure asrequired in order to maintain a selected rate of ow until thetemperature and injection pressure have increased to magnitudes at whichthe vertical fracture is closed by the thermal expansion of its wallsand a horizontal fracture is formed and extended until it interconnectsa pair of wells.

6. A process for producing petroleum from a viscous petroleum reservoirthat is substantially impermeable at reservoir temperature, issubstantially incompetent at a temperature that thermally mobilizes thereservoir petroleum and is penetrated by at least one pair of Wells thatare interconnected by a horizontal fracture extending through thereservoir, which process comprises:

(a) heating aqueous liquid at a surface location and circulating theheated liquid through the fracture between said pair of wells whilemaintaining a back pressure high enough to provide a pressure in thefracture that is suicient to separate the walls of the fracture so as tocause ow at a rate adequate for transferring heat from the surfacelocation to the openings into the production well;

(b) increasing the temperature of the circulating heated aqueous liquiduntil yiluid flowing into the production well has a temperature thatthermally mobilizes the reservoir petroleum while increasing thesoftness of the aqueous liquid to the increasing extent required toprovide a liquid that is nonscaling at the temperature to which theliquid is being heated;

(c) forming a permeable path within the reservoir by entraining thethermally mobilized reservoir petrole- 9 l0 um in the circulating heatedaqueous liquid and thus References Cited `converting a portion of thereservoir to a layer of UNITED STATES PATENTS petroleum-depletedpermeable sand; (d) reducing the pressure within the reservoir to less3,237,692 3/1966 Wallace et al 16611 X than overburden pressure andcirculating steam 5 3,342,258 9/196'7 Prats -7 166-11 through thereservoir at a temperature at which said 31360045 '12/ 1967 Samounan166`11 reservoir petroleum is mobilized and entrained; and (e)recovering said petroleum from fluid that has cir- CHARLES E' o CONNELLPr'mary Examiner' culated through the reservoir. I. A. CALVERT,Assistant Examiner.

1. A PROCESS FOR PRODUCING PETROLEUM FROM A VISCOUS PETROLEUM RESERVOIRTHAT IS SUBSTANTIALLY IMPERMEABLE AT RESERVOIR TEMPERATURE, ISSUBSTANTIALLY INCOMPETENT AT A TEMPERATURE THAT THERMALLY MOBILIZES THERESERVOIR PETROLEUM AND IS PENETRATED BY AT LEAST ONE PAIR OF WELLS THATARE INTERCONNECTED BY A HORIZONTAL FRACTURE EXTENDING THROUGH THERESERVOIR, WHICH PROCESS COMPRIESE: (A) HEATING AQUEOUS LIQUID AT ASURFACE LOCATION AND CIRCULATING THE HEATED LIQUID THROUGH THE FRACTUREBETWEEN SAID PAIR OF WELLS WHILE MAINTAINING A BACK PRESSURE THAT ATLEAST SUBSTANTIALLY EQUALS THE OVERBURDEN PRESSURE AND AN INJECTIONPRESSURE THAT CAUSES FLOW AT A RATE ADEQUATE FOR TRANSFERRING HEAT FROMTHE SURFACE LOCATION TO THE OPENINGS INTO THE PRODUCTION WELL; (B)INCREASING THE TEMPERATURE OF THE CIRCULATING HEATED AQUEOUS LIQUIDUNTIL FLUID FLOWING INTO THE PRODUCTION WELL HAS A TEMPERATURE THATTHERMALLY MOBILIZERS THE RESERVOIR PETROLEUM WHILE INCREASING THESOFTNESS OF THE AQUEOUS LIQUID TO THE INCREASING T EXTENT REQUIRED TOPROVIDE A LIQUID THAT IS NONSCALLING AT THE TEMPERATURE TO WHICH THELIQUID IS BEING HEATED; (C) FORMING A PERMEABLE PATH WITHIN THERESERVOIR BY ENTRAINING THE TERMALLY MOBILIZED RESERVOIR PETROLEUM INTHE CIRCULATING HEATED AQUEOUS LIQUID AND THUS CONVERTING A PORTION OFTHE RESERVOIR TO A LAYER OF PETROLEUM-DEPLETED PERMEABLE SAND; (D)REDUCING THE PRESSURE WITHIN THE RESERVOIR TO LESS THAN OVERBURDENPRESSURE AND CIRCULATING STEAM THROUGH THE RESERVOIR AT A TEMPERATURE ATWHICH SWAID RESERVOIR PETROLEUM IS MOBILIZED AND ENTRAINED; (E)RECOVERING SAID PETROLEUM FROM FLUID THAT HAS CIRCULATED THROUGH THERESERVOIR.